I don't know the exact dates when things were changed but I have several parts that I ahve picked up over the last year or so and I see many design changes in several parts. My 54xxx was I think a second run. I cracked the slide(too many +p's) and sent it back. they gave me one that was very different. the extractor was different, barrel and quality of the mill on the slide (a little bit better). I also found a slide assembly for sale and bought it. It was also different. Anyway, since I had so many extras, I started to measure everything with calipers. I got a good tight combo with the mix of parts. I used a new older style barrel. I used the newer shaped extractor, newer firing pin and spring, ssteel guide rod and the slide that Keltec sent me for the replacement of mine. I got a good,fairly tight lockup. I noticed that the pistol rattled less when I did the tight fit combo(barrel and slide). The accuracy was vastly improved. The older parts(original 55xxx) we're real sloppy and would group irratically every session. Now the sucker groups 2-3 inches from 10 yds. That's real good since the other combo would group more like a foot! It's probably just me! But I feel better with the current setup. So good that I took out the file and melted the slide and am now waiting for the pzrkerized to come back. I did the jack fuselier nickle job on another slide(the one that broke) and loved it too but decided on the stealth look this time. In any case, the factory will switch any parts that you have probs with...they are the coolest folks that I've had pleasure of doing business with...put it this way, you won't be dissatisfied with the Keltec or the company, great pistol. With sigs,glocks and para, that sit in my house, the keltec gets the most attention.